Process of growing cubic zinc sulfide crystals in a molten salt solvent



March 1968 R, E. JOHNSON ETAL 3,374,067

PROCESS OF GROWING CUBIC ZINC SULFIDE CRYSTALS IN A MOLTEN SALT SOLVENT Filed Dec. 8, 1965 INVENTOR ROWLAND E. JOHNSON SIDN Y 6. PARKER ATTORNEY United States Patent M 3,374,067 PROCESS OF GROWING CUBIC ZINC SULFIDE CRYSTALS IN A MOLTEN SALT SOLVENT Rowland E. Johnson and Sidney G. Parker, Dallas, Tex.,

assignors to Texas Instruments Incorporated, Dallas,

Tex., a corporation of Delaware Filed Dec. 8, 1965, Ser. No. 512,400 Claims. (Cl. 23-305) This invention relates to a method for growing zinc sulfide crystals, and more particularly to a method of growing zinc sulfide crystals that have a cubic structure.

It is known in the art that cubic zinc sulfide crystals may be obtained by vapor or chemical transport methods, but the resulting crystals are generally small and often polycrystalline.

An object of the present invention is to provide a method for growing zinc sulfide crystals of cubic structure that are comparatively monocrystalline.

A further object is to provide a method for growing such crystals that are comparatively large. Yet a further object is to provide a rather simple, yet effective method for achieving the preceding objects.

In accordance with the present invention, a method is provided for growing zinc sulfide crystals which comprises placing a solution of zinc sulfide in contact with an excess quantity of zinc sulfide raw material in a first region at a temperature T and contacting the solution with a second region displaced from the first region while maintaining the solution in the vicinity of the second region at a temperature T The temperature T is greater than T and T is less than about 1000 C., the transition temperature from the hexagonal to cubic crystal structure for zinc sulfide. Zinc sulfide crystals grow in the second region.

In a preferred embodiment of the present invention, a molten salt is provided as the solvent for the zinc sulfide, e.g., molten potassium chloride.

For a more complete understanding of the present invention and for further objects and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawing in which:

The figure is a schematic cross-sectional elevational view of crystal-growth apparatus operating in accordance with the present invention.

In the figure a system is illustrated in which zinc sulfide crystals are being grown in accordance with the present invention. Therein a quartz container 11 is utilized as a crystal growth container. The quartz container 11 is constricted in its central region 13 to provide, in efiect, intermediate baflie means. The chamber portions or regions 15 and 17 inter-communicate through the narrowed throat in the container 11 defined by constricted region 13.

In operation, the container 11 carries a molten salt 18 which is saturated with zinc sulfide. A supply of zinc sulfide powder 19 is disposed within the chamber. region 15. Conventional resistance heating means 21 and 23 maintain the zinc sulfide carrying molten salt at a temperature T; in the chamber region 15, and conventional resistance heating means 25 and 27 maintain the molten salt at a temperature T toward the opposite end of the container, i.e., in region 17.

With T lower than T by several degrees, zinc sulfide powder 19 is, in effect, transferred via the molten salt to the lower temperature region T where crystallization occurs. Accordingly, zinc sulfide crystals 29 are grown in the region 15 of chamber 11.

For best results, a zinc sulfide seed crystal is utilized in region 17 in order to promote nucleation and good crystal growth.

3,374,067 Patented Mar. 19, 1968 By the procedure just described, zinc sulfide (e.g., powder or chunks of zinc sulfide) is transferred through a solution phase, into a region where the solution carrying it is too saturated to maintain it in a solution. The product deposited in such region is found to be largely monocrystalline in nature, with the crystals being of cubic form and of substantial size and quality.

The temperatures at which the molten salt is maintained within the chamber 11 may vary over a wide range, however it is important that maximum temperature in the region 17 where crystal product is to be formed not exceed about 1000 C. since at about that temperature the cubic form of zinc sulfide crystal changes to the hexagonal form. Of course, it is necessary that temperatures be sutficient to maintain the salt which serves as a solvent in the molten state.

A preferred salt for the solvent is potassium chloride. The temperature at which potassium chloride melts is in the vicinity of 790 C. Accordingly, when potassium chloride is used the minimum temperature should be over 7 this value somewhat in order to maintain the system in fluid condition. With potassium chloride as the molten salt, preferably the temperature T ranges from about 800900 C., with the temperature T ranging from about 790890 C. The difference in temperatures T and T may vary over a considerable range, however about 5 10 is quite satisfactory.

The present invention is not limited to the use of potassium chloride as the solvent for zinc sulfide. The requirements for a satisfactory solvent are that it can be maintained as a liquid below the transition temperature of about 1000 C. in which the cubic form of zinc sulfide crystal changes to the hexagonal form; that it be generally inert chemically with respect to the zinc sulfide which it carries; that it be at least a reasonably good solvent for zinc sulfide; and that it have a solubility curve with respect to zinc sulfide that rises with rising temperature. Exemplary of materials which can be used in place of potassium chloride are sodium chloride, sodium bro mide, sodium iodide, calcium chloride, potassium s'ulfide, and zinc chloride.

The following specific examples of practice of the present invention are offered by way of example and are not intended to be taken as limiting.

EXAMPLE 1 A quartz tube of 20 millimeters inside diameter and one millimeter wall thickness is fused at one end and is formed in a central region to provide a constricted portion such as 1 3 in FIGURE 1. The throat defined at the constricted region has a greatest dimension of about five millimeters at the throats narrowest cross section.

A potassium chloride solution saturated with zinc sulfide at a temperature of 800 C. is prepared. This requires about 15 grams of zinc sulfide per grams of potassium chloride. In the size tube utilized in this example, 50 grams of potassium chloride are used. Accordingly, approximately seven and one-half grams of zinc sulfide are needed to saturate the potassium chloride solution prepared at the temperature of 800 C.

The vessel containing the saturated solution at 800 C. is quenched to bring the contents to room temperature. The resulting solidified potassium chloride-zinc sulfide mixture is ground to a finely divided solid.

A seed crystal of zinc sulfide is placed adjacent the sealed end of the tube so that it lies between the constricted region in the tube and such sealed end. The tube is loaded with the ground potassium chloride-zinc sulfide mixture referred to in the preceding paragraph. Thereafter, ten grams of zinc sulfide powder (about five micron average particle size) are placed in the end of the quartz tube which is separated from the seed crystal by the constricted region. The open end of the quartz tube is connected to conventional vacuum pump means and a vacuum of mm. Hg is provided in the tube while heating it to about 150 C. This procedure drives off moisture.

Thereafter the tube is cooled to room temperature while the vacuum is maintained. Then the tube is back-filled with argon or other inert gas to about 200 millimeters of Hg pressure. While in such condition, the system is sealed olf by fusing the end of the quartz tube.

The sealed tube is placed in a cold furnace which is provided with heating coils capable of separate control and close temperature monitoring at the regions of the sealed quartz tube on either side of the constricted central portion. Heating is then conducted to bring the contents of the sealed tube to a temperature of 800 C. This may be performed, for example, over about three hours.

Thereafter, by means of the separate resistance heaters in the region of the quartz tube containing the zinc sulfide powder, a higher temperature is established in such region. This temperature is approximately 810 C. At the same time, the temperature in the opposite end of the sealed quartz tube (i.e., where the seed crystal is contained) is maintained at 800 C.

It is observed that nucleation and crystal growth of zinc sulfide occurs in the region of the seed crystal. By permitting the system to remain at temperatures of 800 and 810 C., respectively in the two regions for a period of five days, the quantity of growth is found to be about one-half gram.

The crystals are recovered by cooling the sealed quartz tube and breaking it open. Thereafter the potassium chloride is leached out by action of a selective solvent (e.g.,

water). The zinc sulfide crystals obtained range as high as about two millimeters by three millimeters in size. While some twinning is observed, the crystals are generally of good quality and of substantially monocrystalline structure.

EXAMPLE 2 The same procedure as for Example 1 is followed, except the seed crystal of zinc sulfide is omitted. Crystal growth is obtained in the region of the quartz container which is maintained at 800 C. The growth over five days is somewhat less in quantity than was obtained in the prior example and the crystal size is not so large; nevertheless a substantial quantity of monocrystalline cubic zinc sulfide crystals is obtained.

EXAMPLE 3 The procedure of Example 1 is repeated, except the two temperatures maintained are 860 and 855 C. instead of 810 and 800 C. Cubic zinc sulfide crystals, in yield and quality comparable to Example 1, are obtained.

EXAMPLE 4 The procedure of Example 1 is repeated, except that (l) the potassium chloride placed in the quartz tube contains no zinc sulfide mixed with it; (2) the quantity of zinc sulfide added to the tube in the region where the higher temperature is to be established is grams instead of the ten grams of Example 1; and (3) no seed crystal is used.

After a short initial period in which the molten salt solution becomes saturated with zinc sulfide, crystallization of zinc sulfide is then observed to occur substantially as in the prior examples.

The present invention is not limited in practice to the container illustrated in FIGURE 1 and discussed above. Any container (reactor) may be used which provides at least two interconnected regions or compartments, one region for the zinc sulfide raw material and the other for zinc sulfide crystal product growth. A horizontal reactor need not even be baflled (e.g., constricted) if it is made long enough, but bafiing is preferred to minimize somewhat the rate of transfer, to provide controlled growth rate of crystal product and to clearly separate the raw material region form the growth region.

If desired, a vertical reactor may be used. This may take the form of an upstanding vessel or tube having a baffle or baffles to divide it into upper and lower regions or compartments. Such a reactor may be used, for example, with zinc sulfide raw material carried in the lower region and with crystal growth conducted in the upper region overlying intermediate baffle means. In such case, the lower temperature needs to be maintained in the upper region and the comparatively higher temperature in the lower region.

It is prefered, but not essential, that the present process operate in an inert atmosphere (e.g., argon) and under anhydrous conditions.

While it is not intended that the present invention be limited by theory, it is believed that a statement of the most probable mechanism is in order to assist those in the art in understanding practice of the invention. It is believed that diffusion of zinc sulfide occurs as a result of the tendency to overcome the concentration gradient established in the solvent. Thus, zinc sulfide tends to move from the high temperature region to the low temperature region. In the low temperature region, the solution is saturated and crystallization occurs. In the high temperature region, the dilfusion from that region makes it possible for more zinc sulfide to go into solution. Accordingly, the net result is a mass transfer from the raw material, via the solution, to the product-collection region.

Having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art and it is intended to cover such modifications as fall within the scope of the appended claims.

What is claimed is:

1. A method of forming cubic crystals of zinc sulfide in a molten solvent comprising, placing a solution of zinc sulfide in a molten salt selected from the group consisting of potassium chloride, sodium chloride, sodium bromide, sodium iodide, calcium chloride, potassium sulfide, and zinc chloride, in contact with an excess quantity of zinc sulfide raw material in a first region at a temperature T contacting said solution with a second region displaced from said first region while maintaining said solution in the vicinity of said second region at a temperature T said temperature T being greater than said temperature T and T being less than about 1000 C., but above the melting point of the solvent, whereby the raw material in said first region is in eifect transported by the medium of said solution to crystallize out said zinc sulfide and to form cubic crystal zinc sulfide product in the said second region.

2. The method of claim 1 wherein a seed crystal is disposed in said second region to promote crystallization and wherein said second region and said first region are separated by bafile means.

3. The method of claim- 2 wherein the difference between said temperatures T and T is about 510 C.

4. A method of forming cubic crystals of zinc sulfide in a molten solvent, comprising establishing a saturated solution of zinc sulfide in a molten salt selected from the group consisting of potassium chloride, sodium chloride, sodium bromide, sodium iodide, calcium chloride, potassium sulfide, and zinc chloride in a reactor in contact with both a product-collection region and a raw materialcontaining region, said product-collection region and said raw material-containing region being spaced apart,

providing zinc sulfide raw material in said raw materialcontaining region, and

maintaining said solution at a temperature T in said raw material-containing region while maintaining said solution at a temperature T in said productcollection region, said temperature T being greater than said temperature T and said temperature T being less than about 1000 C., but above the melting point of the solvent, whereby cubic zinc sul fide crystal is deposited in said product-collection region.

5. The method of claim 4 wherein a seed crystal is disposed in said productcollection region to promote crystallization.

6. The method of claim 4 wherein said product-collection region and said raw material-containing region are separated by bafiie means.

7. The method of claim 4 wherein said zinc sulfide is carried in solution by molten potassium chloride.

8. The method of claim 7 wherein said reactor is provided with an inert, substantially anhydrous atmosphere.

9. The method of claim 8 wherein said temperature T is between about 800 and 900 C. and said temperature T is between about 790 and 890 C.

6 10. The method of claim 9 wherein the difference in said temperatures T and T is between about 5 and 10 C.

References Cited UNITED STATES PATENTS 2,020,? 13 11/1935 Holstein 23-135 2,130,382 9/1938 Copeland 23135 3,146,204 8/1964 Auen 23-135 OTHER REFERENCES Semi-Conductors by Hannay, Bell Telephone Laboratories, Reinhold Publishing Corporation, New York, 1959, pp. 127-132.

NORMAN YUDKOFF, Primary Examiner. G. P. HINES, Assistant Examiner. 

1. A METHOD OF FORMING CUBIC CRYSTALS OF ZINC SULFIDE IN A MOLTEN SOLVENT COMPRISING, PLACING A SOLUTION OF ZINC SULFIDE IN A MOLTEN SALT SELECTED FROM THE GROUP CONSISTING OF POTASSIUM CHLORIDE, SODIUM CHLORIDE, SODIUM BROMIDE, SODIUM IODIDE, CALCIUM CHLORIDE, POTASSIUM SULFIDE, AND ZINC CHLORIDE, IN CONTACT WITH AN EXCESS QUANTITY OF ZINC SULFIDE RAW MATERIAL IN A FIRST REGION AT A TEMPERATURE T1, CONTACTING SAID SOLUTION WITH A SECOND REGION DISPLALCED FROM SAID FIRST REGION WHILE MAINTAING SAID SOLUTION IN THE VICINITY OF SAID SECOND REGION AT A TEMPERATURE T2, SAID TEMPERATURE T1 BEING GREATER THAN SAID TEMPERATURE T2 AND T2 BEING LESS THAN ABOUT 1000*C., BUT ABOVE THE MELTING POINT OF THE SOLVENT, WHEREBY THE RAW MATERIAL IN SAID FIRST REGION IS IN EFFECT TRANSPORTED BY THE MEDIUM OF SAID SOLUTION TO CRYSTALLIZE OUT SAID ZINC SULFIDE AND TO FORM CUBIC CRYSTAL ZINC SULFIDE PRODUCT IN THE SAID SECOND REGION. 